1. Field of the Invention
This invention is directed to an apparatus and method for detecting reflections, and in particular to a wavelength specific optical reflection meter/locator for WDM (wavelength division multiplexed) systems.
2. Background Art
In optical amplified systems, the reflection of a significant portion of the light leaving via a given fiber may cause problems with detection of the loss of the input signal on that fiber. If the reflected outgoing light could be distinguished from the desired input signal, then appropriate alarms or control actions could be initiated. The outgoing light, that is then reflected, could be amplified signal and amplified spontaneous emission (ASE), as in the case of a bi-directional system, or could be just ASE, as in the case of a unidirectional system. Or, the outgoing light could be a combination of signals and ASE from both directions in the case where there are more complex optical path reflections.
Especially in multi-channel bi-directional optical system, reflections can cause optical amplifiers to oscillate, despite the optical isolators that may be present. This oscillation path can involve more than one optical amplifier in the system and be quite complex. The detection of reflections is important in that it helps to prevent oscillations in bidirectional optical amplifiers, as also it provides the power of the signal propagating in the direction of the reflection. Moreover, in multi-channel systems, where signals of different wavelengths travel in different optical paths, the reflections are also different for each optical path. Location of the main source of a particular reflection is also desirable for trouble-shooting purposes.
Measurement of the strength of reflections is presently done with an optical time domain reflectometer (OTDR) that sends strong short pulses of light down a fiber and measures the signal returned. This is an accurate method, but the OTDR is a relatively large and expensive piece of test equipment that can not easily be used when there is traffic on the fiber.
Optical frequency domain reflectometry (OFDR) may also be used to detect faults in an optical link. According to this method, the optical frequency is varied and optically coherent detection is used, or an optical source is modulated with a constant amplitude tone that is swept in frequency.
Reflections may be detected by modulating the laser pump of an optical amplifier in the optical path of interest, which modulates all signals output by that particular amplifier, as disclosed in the U.S. patent application Ser. No. 08/588,176 (O'Sullivan et al.), filed on Jan. 18, 1996 and assigned to Northern Telecom Limited. The dither method requires controlling the dither at each amplifier site, and has no consideration of the wavelength dependency of the reflection.
Still another prior art method is to measure the amount of DC light reflected back via a four port coupler. However, this method does not stimulate or consider the AC portion of the signals, and also does not provide for locating the fault. The DC reflection is used to determine if a large reflection from a broken fiber or open connector is present at the output of a network element, such as an optical amplifier, so as to shutdown the output of the faulty network element for safety. In addition, this has been known to falsely trigger from low level reflection due to Raleigh scattering in the fiber. In addition, this method cannot be used in bidirectional systems.
The prior art generally fails to provide a method for easy and accurate detection of reflections in optical systems, and for distinguishing between equipment failures and lack of an input signal.
There is a need to distinguish optical reflections from valid inputs when isolating a cable break in a bidirectional WDM optically amplified system.